The multi-band OFDM communication is based on multi-point to multi-point type of communication. In this type of communication a set of transceivers are coupled together defining a local network, e.g. a wireless local area network (WLAN). Several transceivers belonging to the same wireless local area network use the same frequency spectrum and thus use the same data transmission channel by means of time domain sharing. This method is referred to as TDMA (TDMA=time domain multiple access).
In this communication system at any specific time only one transceiver is allowed to transmit data. Accordingly, the data communication between the different transceivers is burst like, that is the transmitting transceiver sends the data information to the receiving transceiver by means of several data transmission bursts. For supporting the receiving transceiver to identify these data transmission bursts and for extracting the delivered data information therein the transmitting transceiver sends a predefined preamble preceeding the data portion of the data transmission burst. Every receiving transceiver within the wireless local area network has no previous knowledge of the received bursts. This lack of knowledge includes the timing of the burst, the identity of the sender, and consequently the received signal level.
However, the receiving transceiver comprises a preamble detection unit that identifies the preamble and thus identifies the data transmission bursts. The transceiver uses further the preamble for estimating data transmission and channel parameters such as channel response and carrier and timing offsets that are needed for the extraction of the data information extraction. The predefined preamble preceding every data transmission bursts assists the receiver to detect the existence of this preamble and to extract parameters that enable demodulation, decoding and data extraction. Among the extracted parameters within the preamble there are parameters that are significant for the tuning of the analog front end (AFE) of the transceiver such as the level of the AGC unit (AGC=automatic gain control) and the timing of a so-called frequency hopping.
To perform this task an interface arranged between the AFE-unit of the transceiver and the base-band unit of the transceiver is needed which is designed to enable fast detection and acquisition of the necessary parameters within the preamble detection within a relatively short time period.
A radio communication link consists at least of the following communication (interface) units: a transmitter performing the pre-equalization and a receiver performing an automatic gain control (AGC), an analog signal level detection, a DC-offset cancellation. Moreover, both units (i.e. the transmitter and the receiver) have to perform frequency hopping which is in particular difficult for the receiving unit. To perform the above mentioned task, consequently it is a challenge to fulfill the following requirements of the transmitter and receiver, respectively.
Pre-Equalization (within Transmitter):
In terms of the transceiver performance—that is the achievable data rate and/or the achievable range—it is desired to transmit as much power as allowed. However, the regulation authorities such as the FCC in the USA limit the allowed transmitted power spectrum density (PSD). Therefore, it is desired to transmit the maximum power in each band without violating the regulation limit.
The analog transmitting path including the antenna is characterized by a different gain in each transmitted frequency band. Therefore, for obtaining improved performances of the data transmission it is necessary and advantageous to provide a transmitter having a different gain for every transmitted frequency band. For the realization of this transmitter a fast data communication between the two units of the communication system is needed.
AGC (within Receiver):
A similar, however more complex problem exists with the receiving path and especially with the AGC-unit therein. Here, the problem is more severe since the received signal level is not known in advance, but should be tuned within the preamble of the received data transmission burst. The base-band unit of the transceiver should carry out several tasks within processing of the preamble including preamble detection, timing acquisition, carrier offset acquisition, etc. This should be done on the one hand within a time limited preamble and on the other hand under very challenging noise conditions. The conditions are severe since the UWB typically relies on processing gain of some spreading modulation resulting in data rates that are significant lower than the utilized bandwidth.
Analog-Signal-Level Detection (within Receiver):
An additional challenge for the AGC mechanism is the fact that signal measurement at the digital domain—that is after the analog to digital conversion (ADC) of the transmitted signal—does not necessarily determine the optimal setup of the programmable gain amplifiers (PGA) along the receive path within the AFE-unit. The reason for this ambiguity results from the mutual detection of the in-band signal and an attenuated out of band signal without the ability to differentiate between these two signals. Miss-selection of the PGA-levels may result in signal compression along the receiving path.
DC Offset-Cancellation (within Receiver):
Sometimes the AFE-receiver causes a DC-offset at the ADC input to a greater or lesser extent. However, such a DC offset either reduces the effective dynamic range of the ADC or potentially saturates the signal in a way that even no signal is received. To prevent this effect there is a need for a DC offset cancellation in the receiving path. However, treating a DC offset for the multi-band OFDM is a challenging task as a result of the very fast band hopping (the DC offset is typically band dependent) and since the offset depends on the setup of the programmable gain amplifiers (PGA) along the receiving path and should therefore be acquired on a burst basis within the preamble signal.
Hopping Commands:
The base-band unit uses the preamble signal to identify the hopping time of the received signal, and additionally selects the hopping timing for enabling good performance. Therefore the base-band unit should command the AFE-unit by means of a hopping signal. Here, a main challenge lies in the provision of an accurate hopping time with a low rate signal.
As mentioned above, UWB-communication systems offer in contrast to previous digital communication systems significant higher data rates, however at the price of larger bandwidths, shorter settling times and higher frequency hopping rates. Far these increased requirements for of the over-all system more sophisticated interfaces between the individual parts of this system are necessary.
A transceiver typically represents a fundamental part of a communication system. The digital transceiver comprises a base-band processor (base-band, BB-unit) and a radio transceiver unit (RF-unit). The base-band processor manages the base-band data stream and controls the function of the RF-unit. The RF-unit requires a special configuration and control apparatus for its function. In some implementations the RF-unit and the BB-unit of a digital transceiver are implemented on separate chips. This has both, advantages and disadvantages. In any case this also requires a sophisticated and well-defined interface between the RF-unit and the BB-unit in order to not reduce the required system performance.
Even if the interface between the RF-unit and the BB-unit—hereinafter referred to as RF-base-band-interface—is similar to single-chip solutions being entirely on the same chip it requires careful considerations. For previous digital communication systems the performance requirements were much more relaxed and therefore a larger variety of different data and control interfaces emerged.